The long term goals of our work are to determine the mechanisms by which T lymphocytes are activated by very small numbers of MHC-peptide complexes (antigen) on the surface of antigen presenting cells. This sensitivity of the T cell to antigens determines the thresholds for processes such as negative and positive selection as well as activation of mature T cells and thus may play an important role in tolerance and autoimmunity. The major focus of this proposal is on the specific roles of adhesion molecules in the extraordinary sensitivity of T cells to antigen which is evident both in vitro and in vivo. T cells respond to as few as 10-100 MHC peptide complex per antigen presenting cell. The solution affinity of T cell antigen receptors for antigenic MHC-peptide complexes is low. Recent work from our lab shows that the mechanism by which T cell adhesion molecules with low solution affinity function to form many bonds in contact areas is to align the apposing membranes with nanometer precision, thus increasing the effective concentration of the receptor and ligand. We hypothesize that sensitive antigen recognition by T cells will depend upon the alignment of the T cell and antigen presenting cell membrane so that the gap between the membranes is 15 nm, the distance between cells that is spanned by the T cell receptor (TCR)- MHC antigen complex. The hypothesis predicts that adhesion mechanisms, such as CD2/ligand interaction, which are predicted to create a gap of 15 nm between the T cell and antigen presenting cell, will be sufficient to promote efficient TCR engagement. Once engagement occurs, adhesion molecules may also contribute to amplification of signals by phosphatase exclusion and recruitment of cytoplasmic proteins to the site of TCR engagement. We will test these hypotheses in 4 Specific Aims. In Alm 1 we will determine whether close contact is sufficient for sensitive antigen recognition. In Aim 2 we will determine whether disrupting close contacts decreases sensitivity. In Aim 3 we will determine whether alternative splicing of ICAM-1 regulates contact area topology and sensitivity to antigen. In Aim 4 we will determine whether the large glycoprotein CD45 is excluded from close contacts and if the exclusion of the phosphatase domain is required for sensitive antigen recognition. The results will contribute to our basic understanding of the antigen recognition process and may define novel therapeutic strategies for autoimmune disease and transplantation medicine based on changing the sensitivity of T cells to antigens through disruption or stabilization of close contact area.